Protein tyrosine kinase (PTKs) are enzymes which, in conjunction with ATP as a substrate, phosphorylate tyrosine residues in peptides and proteins. These enzymes are key elements in the regulation of cell signaling including cell proliferation and cell differentiation. PTKs comprise, inter alia, receptor tyrosine kinases (RPTKs), including members of the epidermal growth factor kinase family (e.g., HER1 and HER2), platelet derived growth factor (PDGF), and kinases that play a role in angiogenesis (Tie-2 and KDR); and, in addition, non-receptor tyrosine kinases, including members of the Sky, JAK and Src (e.g., Src, Fyn, Lyn, Lck and Blk) families (see Bolen, J. B. Rowley, R. B., Spana, C., and Tsygankov, A. Y., "The src family of tyrosine protein kinases in hemopoietic signal transduction", FASEB J., 6, 3403-3409 (1992); Ulrich, A. and Schlessinger, J., "Signal transduction by receptors with tyrosine kinase activity", Cell, 61, 203-212 (1990); and Ihle, J. N., "The Janus protein tyrosine kinases in hematopoetic cytokine signaling", Sem. Immunol., 7, 247-254 (1995)).
Enhanced activity of PTKs has been implicated in a variety of malignant and nonmalignant proliferative diseases. In addition, PTKs play a central role in the regulation of cells of the immune system. PTK inhibitors can thus impact a wide variety of oncologic and immunologic disorders. Such disorders may be ameliorated by selective inhibition of a certain receptor or non-receptor PTK, such as Lck, or due to the homology among PTK classes, by inhibition of more than one PTK by an inhibitor.
A PTK of particular interest is Lck which is found in T cells where it is involved in phosphorylating key protein substrates. It is required for productive antigen receptor signaling and cell activation. In the absence of Lck activity, the T cell receptor (TCR) zeta chain is not phosphorylated, the kinase ZAP-70 is not activated, and Ca.sup.2+ mobilization essential for T cell activation does not occur (see Weiss, A. and Littman, D. R. "Signal transduction by lymphocyte antigen receptors", Cell, 76, 263-274 (1994); Iwashima, M., Irving, B. A., van Oers, N. S. C., Chan, A. C., and Weiss, A., "Sequential interactions of the TCR with two distinct cytoplasmic tyrosine kinases", Science, 263, 1136-1139 (1994); and Chan, A. C., Dalton, M., Johnson, R., Kong, G., Wang, T., Thoma, R., and Kurosaki, T., "Activation of ZAP-70 kinase activity by phosphorylation of tyrosine 493 is required for lymphocyte antigen receptor function", EMBO J., 14, 2499-2508 (1995)). Inhibitors of Lck are thus useful in the treatment of T-cell mediated disorders such as chronic diseases with an important T cell component, for example rheumatoid arthritis, multiple sclerosis and lupus, as well as acute diseases where T cells are known to play an essential role, for example acute transplant rejection and delayed-type hypersensitivity (DTH) reactions.